Levels of carbon dioxide (CO2) and the other major greenhouse gases reached record highs in 2025, locking in more long-term warming for the planet, the World Meteorological Organization (WMO) said on Tuesday in its annual Greenhouse Gas Bulletin.

Source: WMO, Greenhouse gas levels once again reach record high, Oct 27 2026

The globally averaged concentration of CO2 was 426.8 parts per million (ppm) in 2025, or 153 per cent of the 1750 pre-industrial baseline, according to measurements from the WMO's Global Atmosphere Watch network. Methane reached 1,948 parts per billion and nitrous oxide 339.1 parts per billion — both also records, at 267 per cent and 126 per cent of their pre-industrial levels respectively, the Bulletin (No. 22) reported.

Source: WMO Greenhouse Gas Bulletin No. 22

426.8 ppmGlobal average CO2 concentration in 2025 — 153% of the pre-industrial level, WMO
1,948 ppbMethane concentration in 2025, a record at 267% of its pre-industrial level
339.1 ppbNitrous oxide concentration in 2025, a record at 126% of its pre-industrial level
377.5 ppmCO2 level when the Bulletin was first published in 2004 — nearly 50 ppm lower than today

"Another year, another new record. Greenhouse gases are trapping an unprecedented amount of heat in the atmosphere, providing energy for more extreme weather and associated risks to our economies and societies," WMO Deputy Secretary-General Ko Barrett told journalists in Geneva. "Every additional part per million matters because it traps additional heat. In effect, we are supercharging the ocean and the atmosphere. That means that heat waves, tropical cyclones, and rainstorms are intensifying faster than our historical baselines prepared us for," she said, adding that the past 11 years have been the warmest on record.

Source: UN News, Oct 27 2026, reporting Ko Barrett's Geneva briefing

Growth slowed slightly — but El Niño could push it up again

CO2 rose by 2.8 ppm between 2024 and 2025 — a smaller jump than the exceptional 3.5 ppm surge of 2023–2024, but still slightly above the average annual growth rate of the last decade, the WMO said. Its senior scientific officer Oksana Tarasova said the earlier surge coincided with a strong El Niño that weakened the land carbon sink through drought, drier vegetation and wildfires, and warned that concentrations can be expected to rise further in 2026 as fossil-fuel burning continues and a powerful new El Niño develops. About half of all CO2 emitted each year stays in the atmosphere; the rest is absorbed by land ecosystems and the oceans. The Bulletin measures concentrations — the gas accumulated in the atmosphere — not annual emissions.

Source: New Scientist, Oct 27 2026, on the WMO Bulletin findings

The Bulletin also flagged two lesser-known gases linked to the energy transition itself. Sulphur hexafluoride (SF6), used as a spark suppressant in electricity distribution, rose by a record amount in 2025; it lasts more than 1,000 years in the atmosphere and has a 100-year warming potential about 24,300 times that of CO2. Molecular hydrogen, increasingly used for energy storage, rose 5.3 per cent between 2010 and 2024, and the WMO said leakage from production, pipelines and storage is a growing concern because of its indirect warming effect. "Both H2 leakage and SF6 persistence represent potential threats that could undermine the climate benefits of the energy transition," the agency said.

The WMO releases the Bulletin each year to give negotiators authoritative data ahead of the UN climate conference; COP31 meets in Antalya, Türkiye, from 9 to 20 November. In South Korea, the Korea Meteorological Administration said on Tuesday that background concentrations of all three gases measured in Korea exceeded the global averages, DongA Science reported.

Source: DongA Science, Oct 27 2026, reporting the KMA announcement on Bulletin No. 22